gination from the posterior end of the inner vesicle; the 

 two sacs are not formed at the same tine, and the left one 

 is always larger that the right. 



In the buds of the Polyclinidae the epicardium is 

 formed in the same 7.'ay; two small diverticx^la, a right and 

 a left one, are given off from the posterior end of the bran- 

 chial sac, from which they afterv/ards become detached. They 

 soon, however, unite to form a single tube, which is contin- 

 ued out into the postabdomen, where it is destined to fur- 

 nish the inner vesicles of the buds produced by transverse 

 constriction of that region of the body. 



The existence of an epicardium in Botryllus is de- 

 nied by Hjort (8), but maintained by Pizon (22). According 

 to the latter the inner vesicle at a very early stage gives 

 off two anterior lateral diverticula, one on each side, which 

 later form the peribranchial cavity, and also two posterior 

 lateral diverticula. These four pouches are at first sepa- 

 rate, but soon the two on each side fise in the middle region 

 of the bud. V^hen the peribranchial cavity is separated from 

 the inner vesicle, the posterior diverticula are cut off at 

 the sane time, and now appear as posterior prolongations of 

 the cloacal cavity, with which they always remain in free 

 communication. They are what Pizon calls the "dive rticules 

 perivisceral^;:" , and in later stages completely envelop the 

 digestive tract. From the fact that these cavities arise 

 as two diverticula from the posterior end of the inner vesi- 

 cle Pizon regards them as homologous with the epicardial 



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